Studies of the induction of recessive lethal mutations in mice by chemical mutagens are being conducted. Backcross progeny between F1 offspring to chemical mutagen treated parents and their F2 daughters are used to detect the presence of recessive lethals. An increase in fetal mortality is used as indicator of the lethal. The induction of recessive lethal mutations by irradiation has previously been demonstrated in the mouse. No adequate study of this class of mutation has been conducted in mice following treatment with chemical mutagens. It is our purpose to investigate whether chemical mutagens will induce recessive lethals in mice and the mode of action and long-term effects of such mutations on mammals.